A common application of ultrasound is for the detection of cracks, corrosion and other similar flaws in metals and welded structures. A particular need is to detect flaws in welded structures. These flaws include such things as lack of fusion planes, porosity, inclusions, and other similar flaws that may compromise the structural integrity of the welded part. The majority of contact testing employs longitudinal waves propagating normal to the test object surface. However, there is often a need for employing shear waves propagating at an angle, for example, for imaging surfaces substantially parallel to the test object surface, discontinuities in weld bevels, e.g., welds with countersinks, or other uneven top surfaces.
Many ultrasound imaging studies have been carried out on welds. A particularly good set of images have been presented in Chapter 4 of Nondestructive Testing Handbook, Second Edition, Volume Seven on Ultrasonic Testing, the disclosure of which is incorporated herein by reference. Images are shown of welds that have regions with a lack of fusion, regions of welds with porosity, and welds showing regions of discontinuities as well as regions of intergranular stress corrosion cracks.
Fatigue failures occur in those locations that are subjected to repeated high stress loadings and are usually associated with areas having stress concentrations, e.g., around fasteners. For example, in the areas around a flat head rivet, the base material features a countersunk hole to accept the rivet head. This countersink leads to several “knife edges” (e.g., the rivet opening with walls normal to the structure surface) that serve to further increase the normal stress concentration caused by the hole alone and thus are particularly prone to fatigue failures. These failures typically begin as a crack that is initiated at one of the knife-edges that then progresses with repeated loading
The prediction of any fatigue crack is an extremely complex combination of stress history and material properties with a large statistical variation expected even under perfect laboratory conditions. Thus, one way to ensure that no cracks are present is to perform an inspection of fasteners in the highly stressed areas, e.g., in highly stressed areas of the airframe.
Present inspection methods employ an ultrasonic testing (UT) transducer. For example, typical portable ultrasound devices allow the operator to make determinations about internal defects by sending out an ultrasound pulse and studying the amplitude and time of returned pulses. In another example, an ultrasonic testing (UT) transducer that is slowly rotated around a fastener can develop a map of the full 360° region. Since a focused beam transducer is used, only a single pencil-point region is checked at a time, and consequently the whole process takes on the order of minutes per fastener. Further, the direct test results (before any post-processing) are signal responses that require an experienced operator to interpret correctly. The combination of long inspection times and high operator skill requirements can limit the overall probability of detection of any cracks or flaws.